Profiling instruments were first developed in the 1930's for the purpose of characterizing surfaces in terms of roughness, waviness and form. In recent years, they have been refined for precise metrology in the measurement and production control of the thin film artifacts which are the building blocks of semiconductor devices. As the semiconductor industry has progressed to smaller dimensions with each new generation of product, the need for more sensitive and precise profiling instruments has grown. As artifacts become smaller, a smaller radius stylus must be used to fully resolve them. But a smaller radius produces higher contact pressure and necessitates use of lower stylus force. The use of very low stylus force renders the instrument more vulnerable to noise generation from roughness of the measured surface and also from environmental sources of vibration. The presence of noise in the output reduces the effective sensitivity of the instrument and compromises the fidelity of its traces. Fidelity is also lost whenever the ratio of stylus pressure to surface yield strength rises to the degree that plastic deformation of the surface occurs and detail of the surface variations is obliterated. Reduction of stylus force is the only solution to this problem.
In U.S. Pat. No. 4,103,542 Wheeler et al., assigned to the assignee of the present invention, disclose a counterbalanced stylus arm, pivoted about a bearing, in which stylus force may be adjusted by moving the counterbalance. Force is measured using a linear variable differential transformer having a core associated with the stylus arm and a coil, through which the core moves, supported independently of the arm. In U.S. Pat. No. 4,391,044 Wheeler discloses a similar stylus arm supported for linear scanning.
It is evident that operation of profilers at very low stylus force is desirable. The present state of the art in commercial profilers allows operation down to 1.0 mg. of force. However, a relatively quiet environment is necessary for good results at that force and such conditions are not always available in the users environment. What is needed is a reduced reaction of the stylus/sensor assembly to the vibration or shock energy pulses which reach it from whatever source.
An object of the invention was to devise a stylus assembly for a profilometer with improved vibration and shock insulation properties.